Transformers are typically employed in electronic trip circuit breakers for current sensing and powering of an electronic trip unit. This is the case, for example, in the implementation described in U.S. Pat. No. 5,089,928, issuing on Feb. 18, 1992 to Durivage, III, et al. and entitled, "Processor Controlled Circuit Breaker Trip System Having Reliable Status Display."
Toroidal transformers are particularly attractive for this application, because they are not easily influenced by adjacent magnetic fields and they tend to have low excitation current compared to other constructions. However, toroidal transformers, due to their geometric shape, are unwieldy to secure in a sufficiently fixed position to protect them from shock and vibration caused by handling and shipping, and magnetic fields. Due to both mechanical and electromechanical forces, the transformer tends to move relative to its mounted position, and this exerts a stress on the lead wires emanating from the toroidal transformer. For instance, because of their shape, toroidal transformers tend to rotate due to vibration and general movement of the mounting structure. After sufficient rotation in one direction, the lead wires emanating from the transformer become the only structure which limits the rotation, and this can damage the lead wires. Furthermore, a current transformer normally needs supplementary insulation to provide sufficient dielectric strength between the bus and the secondary winding.
One method of overcoming these problems is to pot the transformer in a cup with a suitable potting compound, or to pot it directly without the benefit of a cup. The assembly may then be provided with means to secure it in position and the potting, or potting and cup, may provide supplementary insulation as well. The potting process, however, is relatively messy, costly and it increases the overall size of the assembly.
Toroidal transformers are also sometimes attached to printed wiring boards with a generally conical washer and a screw.